With appropriate electrical loading, photovoltaic solid state semiconductor devices, commonly known as solar cells, convert sunlight into electrical power by generating both a current and a voltage upon illumination. The current source in a solar cell is the charge carriers that are created by the absorption of photons. These photogenerated carriers are typically separated and collected by the use of PN or PIN junctions in semiconductor materials. The operational voltage of the device is limited by the dark current characteristics of the underlying PN or PIN junction, among other limiting characteristics. Thus improving the power output performance of any solid state solar cell generally entails simultaneously maximizing absorption and carrier collection while minimizing dark diode current.
Quantum well solar cells seek to harness a wide spectrum of photons at high voltages in a single junction device by embedding narrow energy-gap wells within a wide energy-gap matrix. By avoiding the limitations of current matching inherent in multi junction devices, quantum well waveguide solar cells have the potential to deliver ultra-high efficiency over a wide range of operating conditions. Quantum well solar cells have been demonstrated in a variety of different material systems, and the basic concept has been extended to include quantum dots. Clear improvements in lower energy spectral response have been experimentally confirmed in both quantum well and quantum dot solar cells. However, photon absorption, and thus current generation, is hindered in conventional quantum structured solar cells by the limited path length of incident light passing vertically through the device. Moreover, the insertion of narrow energy-gap material into the device structure often results in lower voltage operation, and hence lower photovoltaic power conversion efficiency.
Optical scattering into laterally propagating waveguide modes provides a physical mechanism to dramatically increase photocurrent generation in quantum well solar cells via in-plane light trapping. The refractive index contrast in a typical quantum well solar cell provides lateral optical confinement and naturally forms a slab waveguide structure. Coupling of normally incident light into lateral optical propagation paths has been reported to lead to increases in the short circuit current of InP/InGaAs quantum well waveguide solar cells coated with nanoparticles. However, maintaining high open circuit voltage remains a universal challenge for all quantum well and quantum dot solar cell devices. It is therefore desirable to provide a device with a novel material structure to achieve high open circuit voltages.